(1) Field of the Invention
This invention relates to a process for producing a carbon fiber of enhanced oxidation resistance.
(2) Description of the Prior Art
In general, a carbon fiber is manufactured by a process wherein a precursor fiber such as an acrylonitrile polymer fiber, a regenerated cellulose fiber, a pitch fiber or the like is heated at a temperature of 200.degree. to 350.degree. in an oxidizing gas atmosphere thereby to be oxidized; then, the oxidized fiber is carbonized at a temperature of at least 700.degree. C. in a non-oxidizing gas atmosphere. The resulting carbon fiber is depending upon the temperature at which the fiber is heated in the final step, classified into two types, i.e. a carbon fiber in a narrow sense and a graphite fiber. A carbon fiber in a narrow sense means a carbon fiber obtained by heating the oxidized fiber at a temperature of 700.degree. C. to 1,600.degree. C., thereby to be carbonized; and a graphite fiber means a fiber obtained by further heating the carbonized fiber at a temperature of 1,600.degree. to 3,000.degree. C., thereby to be graphitized. Due to the difference in heating temperature, the carbon fiber and the graphite fiber are different in their mechanical properties as well as their specific volume resistance and nitrogen content. The graphite fiber usually exhibits a larger modulus of elasticity, a smaller tensile strength and worse adhesive properties for composite matrixes such as resins and carbon, than those of the carbon fiber.
An illustration of graphitization of carbon fibers is given in U.S. Pat. No. 4,001,382. It is stated in column 4, lines 28 through 31 of this patent that carbon fibers are heated generally to a temperature of 2,000.degree. C. to 3,500.degree. C. in order to graphitize the carbon fibers. By graphitization conducted at such a high temperature, most of the impurities contained inside the carbon fibers are expelled therefrom, and thus, the resultant graphite fibers exhibit enhanced oxidation resistance. However, the graphite fibers are very costly and, as mentioned above, poor in tensile strength and adhesive properties for composite matrixes. Thus, the graphite fibers have only limited applications.
With respect to carbon fibers, many proposals have been heretofore made in order to enhance their tensile strength and modulus of elasticity, for example, in U.S. Pat. Nos. 4,001,382; 4,024,227; 3,993,719 and 4,080,417. The main points in these proposals are as follows. (1) the step of incorporating an acrylonitrile copolymer having comonomer units possessing carboxyl groups, at least a part of which has an alkali metal or ammonium ion introduced therein, in the acrylonitrile polymer to be made to an acrylic fiber precursor; (2) the step of treating a water-swollen acrylic fiber with a aqueous solution of a primary amine or ammonium salt; (3) the step of treating a water-swollen acrylic fiber with an aqueous emulsion of aminosiloxane; or (4) the steps of drawing an acrylic fiber to a great extent and drying the drawn acrylic fiber to an extent such that the water content is less than 4% by weight.
Although tensile strength and modulus of elasticity of carbon fibers can be enhanced by the above-mentioned proposals, oxidation resistance of carbon fibers is not enhanced. It now has been found by the inventors of the present invention that oxidation resistance of carbon fibers greatly varies depending upon the amounts of the particular metal impurities contained in the carbon fibers. That is, carbon fibers containing significant amounts of Na, K, Fe, Cu, Ni, Co, Cr and Mn are poor in oxidation resistance. In the conventional techniques for the production of carbon fibers, including the above-mentioned proposals, no consideration is given for preventing or minimizing the incorporation of the specified metal impurities into the acrylonitrile polymer, the acrylic fiber precursor made therefrom, or the carbon fiber made therefrom over the entire courses spanning from the step of polymerizing acrylonitrile to the step of collecting carbonized fibers. In some cases, the acrylonitrile polymer, the acrylic fiber precursor made therefrom or the carbon fiber made therefrom may contain, in addition to the above-specified metal impurities, other metals such as Zn, Pb, Sn and Hg, and halogens and sulfur. These impurities, other than the above-specified metal impurities, reduce the adhesive properties for composite matrixes, such as resins and carbon, and exert a harmful influence upon the human body. Furthermore, waste gases, generated when acrylic fibers containing halogens and sulfur are carbonized, cause air pollution. However, the impurities, other than the above-specified metal impurities, have little or no influence upon the oxidation resistance of the carbon fibers.
It is to be noted that, in most of the conventional techniques for the production of carbon fibers, the acrylic fibers are inevitably treated with an aqueous solution which contains some of the above-specified metal impurities, and methods to prevent or minimize the incorporation of the specified metal impurities into the acrylic fibers are not considered. For example, when an acrylonitrile copolymer containing comonomer units having carboxyl groups, at least a part of which has ammonium ions introduced therein, as described in U.S. Pat. No. 4,001,382, is extruded into an aqueous coagulating bath to form a fiber, followed by treating the fiber by using an aqueous drawing bath and an aqueous washing bath, an ion exchange reaction rapidly occurs between the ammonium ions and metal impurities present in the aqueous coagulating, drawing and washing baths. This ion exchange reaction is conspicuous particularly when the aqueous coagulating bath has incorporated therein inorganic compunds, such as sodium thiocyanate and other alkali metal salts as coagulating agents. Consequently, the acrylic fiber inevitably contains an increased amount of metal impurities and a reduced amount of the ammonium carboxylate groups, and thus, the resulting carbon fiber is poor in oxidation resistance.